Add type annotations to xtgeo.cube #1431

Add types to the files in xtgeo/cube folder.

Before making the file as done, check that your PR fulfill these requirements. Claim a file by adding a comment to the PR that says what file your working on.

• All function/method parameters are typed

• All return types are annotated

• All mypy ambiguities are resolved

• Doc strings are up to date

• src/xtgeo/cube/_cube_export.py

• src/xtgeo/cube/_cube_import.py

• src/xtgeo/cube/_cube_roxapi.py

• src/xtgeo/cube/_cube_utils.py

• src/xtgeo/cube/_cube_window_attributes.py

• src/xtgeo/cube/cube1.py